In recent years, 3GPP (3rd Generation Partnership Projects) has specified a mobile communication system called a “W-CDMA (Wideband-Code Division Multiple Access) system (or UMTS: Universal Mobile Telecommunications System)” based on CDMA (Code Division Multiple Access) techniques.
FIG. 1 shows a configuration example of the W-CDMA system. As shown in FIG. 1, the W-CDMA system includes a mobile station UE (User Equipment), a radio base station NodeB, a radio control station RNC (Radio Network Controller), and an exchange MSC/SGSN (Mobile Switching Center/Serving GPRS Support Node), thereby configuring a cellular mobile communication system.
Radio waves transmitted from the radio base station NodeB form an area called a “Cell” which serves as a control unit in the cellular system.
In the W-CDMA system, the mobile station UE in a state of not performing communication with its power turned on, i.e., in a standby mode of standing by in preparation for a calling operation by a user or a call receiving operation due to an incoming call, measures electric fields of pilot channels transmitted from the radio base station NodeB and thus autonomously selects an optimum pilot channel.
In the W-CDMA system, a pilot channel is specified by a combination of a frequency band and a spreading code called “Primary Scrambling Code” and is normally linked with a cell on a one-to-one basis.
Specifically, pilot channels which correspond to different channels are respectively provided with different combinations of frequency bands and spreading codes (nevertheless, since the number of combinations of the frequency bands and the spreading codes is limited, the same spreading code may be reused among radio base stations NodeB which are geographically distant from each other).
It conversely means that a cell which transmits a pilot channel having a different combination of a frequency band and a spreading code is a different cell.
For example, the mobile station UE is configured to autonomously select, as the optimum pilot channel, a pilot channel such as one having a characteristic that an electric field intensity or a SIR (signal to interference ratio: a ratio of an electric field intensity of a desired wave to an electric field intensity of an interference wave) is larger than those of others, and one having a characteristic that an electric field loss is smaller than those of others.
Here, the mobile station UE having selected the optimum pilot channel, i.e., the mobile station UE having selected the optimum cell, reads notification information of the cell and enters standing by in the cell.
The mobile station in a standby mode in the cell starts communication by accessing the radio base station NodeB covering the cell or the radio control station RNC to go into a communicating state.
Meanwhile, the mobile station UE in the communicating state measures the electric field intensity of the pilot channel and reports such measurement information to the radio control station RNC either regularly or on a certain opportunity based on a result of a periodical threshold judgment, for example.
Conceivable information as such measurement information is information including the electric field intensity, the SIR, the electric field loss, and the like, as similar to the measurement result in the above-described standby mode.
The radio control station RNC selects the optimum cell on the basis of the measurement information and then notifies the mobile station UE or the radio base station NodeB of the frequency band and the spreading code (such as Primary/Secondary Scrambling Code or Channelisation Code) of a communication channel usable in the cell. Thereby, communication between the mobile station UE and the radio base station NodeB takes place.
In addition, as the mobile station UE moves, the mobile station UE is allowed to change the cell in which the mobile station UE is to perform communication, in the course of communication (handover).
Incidentally, as a type of the radio base station NodeB, a micro radio base station has been progressively introduced in addition to a public radio base station. While the public radio base station covers an outdoor space, the micro radio base station mainly covers an indoor space.
Moreover, an “area-restricted service” has been studied. In the “area-restricted service,” the micro radio base stations are provided in a place such as a home or a small office to minimize a coverage area or to employ an inexpensive IP line for home use for connection with the radio control station RNC. With the “area-restricted service,” a private area (containing a single or plural cells) exclusive for a particular user is formed, where a billing system in the area is discriminated from a normal billing system or an additional function is provided exclusively to the area.
Even when the area-restricted service exclusively for a particular user is provided by the micro radio base stations as in the above case, the mobile station UE in the standby mode selects a cell based on the measurement of the electric field of the pilot channel while the mobile station UE in the communicating state determines a cell, in which the mobile station UE performs communication, with an instruction of the radio control station RNC based on the information on the measurement of the electric field of the pilot channel, as described previously.
For this reason, even located in a geographical vicinity of a micro radio base station, the mobile station UE in some cases stands by or performs communication in a cell formed by a radio base station NodeB in the surroundings and withholds standby or communication in a particular area formed by the micro radio base station, if the electric field intensity of the pilot channel transmitted by the radio base station NodeB in the surroundings is dominant.
As described above, the W-CDMA system according to the related art has a problem that, even when an area-restricted service exclusively for a particular user is provided in a particular area, the mobile station UE does not perform communication in the particular area depending on the condition on the electric field and thereby degrades a service performance.